Clean up E0412 and E0422 explanations

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0412.md

@@ -1,4 +1,4 @@
-The type name used is not in scope.
+A used type name is not in scope.
 
 Erroneous code examples:
 

src/librustc_error_codes/error_codes/E0422.md

@@ -1,4 +1,5 @@
-You are trying to use an identifier that is either undefined or not a struct.
+An identifier that is neither defined nor a struct was used.
+
 Erroneous code example:
 
 ```compile_fail,E0422